By using numerical simulation method, the melting and thermal storage process of molten salt in the thermal storage tank was studied. The results showed that tubes pitches have important influence on the melting process of molten salt in the same thermal storage tank. The tank with 200 mm tubes pitches is of better storage ability than that with 120 mm, and the temperature distribution in molten salt tank is more uniform. The results will provide a reference to the structural design and optimization of the thermal storage tank.